Diverse approaches are available to detect the presence of molecules including, for example, ELISA, PCR and mass spectrometry. Landegren et al (U.S. Pat. No. 7,306,904) describe the detection of analytes in solution using proximity probes. These probes comprise a binding moiety and a nucleic acid molecule. The nucleic acid molecule of one proximity probe may interact with that of another when in close proximity, i.e. when bound to an analyte. The interacting nucleic acid molecules may be amplified to detect the presence of the analyte. However, high background is a common problem that limits the detection of target present at trace levels. For example, in the proximity ligation assay used by Landegren et al, the authors had to use very low concentrations of probes in order to keep the background low. Too low a concentration of the probes is not suitable for binding of the probes to a target molecule, since that would result in low sensitivity for the detection of the target molecules. However, increasing the concentration of the probes results in high background, which is problematic. Therefore, there is a trade-off, and Landegren et al does not differentiate target specific proximity ligation from non-target specific proximity ligation.
Thus there remains a need for a detection system that minimizes background and that forms a stable complex only when bound to a target molecule and wherein the stable complex may be specifically detected.